The present invention relates to a coating device for one-sided or two-sided coating of a travelling web, particularly a paper web. Many devices for sizing or coating of paper webs are known. Many of these are based on transfer of coating material onto the web with the use of metering rolls which either operate immersed in a bath of coating material or are supplied with such material by means of feeding devices extending across the whole width of the web. Such metering rolls that are used in connection with sizers or coating devices of different types are, however, not satisfactory in regard to metering, since they inter alia are strongly viscosity dependent and also result in varying deposition in dependence on web speed. For the purpose of improving metering accuracy so called fast sizers have therefore been developed which are based on metering means consisting of wirewound metering rods which rotate in bearing members extending across the whole width of the web. Such bearing members are necessary since the wirewound metering rods are not selfsupporting. Even if the metering accuracy has been improved by the use of such metering rods this known technique does, however, involve substantial drawbacks among which the following may be mentioned.
The closely wound wire on the metering rod is subjected to a very fast wear in view of the relatively small diameter of the metering rod. The apparatus including its bearing arrangement is, furthermore, mechanically complicated and thereby results in a high investment cost. As will be shown below the first problem of fast wear results in a strongly wear influenced volume of metered material.
The present invention has for its purpose to provide a device by which the prior art drawbacks are eliminated to a great extent and which constitutes a simplified construction and thereby lower costs. Another object of the invention is to provide a coating device resulting in extended life for the metering members thereby offering a more reliable operation.
For these and other objects the present invention provides for a coating device for one-sided or two-sided coating of a travelling web, especially a paper web. This device comprises at least one backing roll and application means cooperating therewith for supplying size or coating agent to the backing roll and, furthermore, means for evening or smoothing of material supplied to the web and, optionally, application means for supplying size or coating agent also to the other side of the web. The coating device is characterized in that the application means comprises self-supporting coating roll, the mantel surface of which cooperating with the backing roll is provided with circumferentially extending, mutually parallel grooves or rifles which provide for volumetric or metered application onto the surface of the backing roll before its contact with the web.
In a preferred embodiment of the coating device according to the invention the coating roll is provided around its mantel surface with a close wire winding, the individual wireturns of which pair-wise form said grooves or rifles. Thus, the wire winding of the coating roll is designed in such a manner that the wire has been wound in one layer with adjacent turns lying in close engagement with each other.
According to an alternative embodiment of the coating device according to the invention the coating roll is around its mantel surface provided with parallel turned or milled grooves. In the same manner as in the wire winding these grooves suitably extend in spiral form around said mantel surface. This spiral-formed arrangement results in simplified machining of the coating roll for providing the desired grooves in its mantel surface.
The diameter of the wire used in the wire winding of the coating roll may vary within wide limits. Normally, it is of the order of millimeters or less, for example some tenths millimeters, such as lying within the range of about 0.2 to about 0.5 mm. The material in the wire is suitably steel or steel alloy.
In the art it is known when using selfsupporting rolls that the contact surface between two rolls, wherein one of the rolls has a somewhat deformable surface, for example of rubber, and the other one has a hard surface, varies along the length of the rolls and is at a minimum at the middle of the rolls. In order to obtain a substantially constant contact surface between the coating roll and the backing roll across the whole width of the web the coating roll may be cambered, i.e. having a diameter decreasing towards the ends thereof and thus the largest diameter in its middle.
An alternative manner of providing for a substantially constant contact surface across the whole width of the web is to subject the coating roll to warping by swinging one end thereof with its other end rotatively and pivotably fixed tangentially along the mantel surface of the backing roll, whereby the contact pressure in the mid section of the rolls increases and the contact surface thereby obtains the desired constant size across the whole width of the web.
It has been indicated above that the coating roll is of a so called selfsupporting type. In order that this criterion shall be met it is suitable that the length-diameter ratio (L/.phi.) of the coating roll does not exceed about 25. It is preferred that said ratio, which also can be regarded to constitute a so called slimness number, lies within the range of about 10 to about 20. At normal web widths of the order of some meters up to about 6 to 7 meters these slimness numbers mean that the diameter of the roll is of the order of a few decimeters and can be up to about half a meter or more.
The coating device according to the present invention can be designed for two-sided coating of a travelling web and may for this purpose comprise two cooperating backing rolls, between which the web is passed, and two coating rolls, each cooperating with its backing roll, designed in accordance with the invention. According to an alternative embodiment for such two-sided coating the device may also comprise a backing roll with an associated coating roll and a blade coater placed on the opposite side of the backing roll, for example of the BILLBLADE.RTM. type.